fix: preserve user-selected half screen mode when clearing filters

[SergioChan]

Summary

Fixes a screen-mode regression when exiting commit filtering.

Closes #5179.

When users start filtering from screenMode: half, clearing the filter should keep the UI in half mode. Instead, Lazygit switched back to normal mode.

Root cause

ModeHelper.ClearFiltering() always converted SCREEN_HALF -> SCREEN_NORMAL on exit, without distinguishing:

• half mode that was entered automatically for filtering, vs
• half mode explicitly configured by the user.

What changed

• Track whether entering filtering actually changed screen mode (normal -> half) using a new screenModeChanged flag in filtering mode state.
• Only restore half -> normal on filter clear when that flag is true.
• Preserve this flag while changing filter type/path during an active filtering session.
• Added a unit test for filtering-mode state reset behavior.

Validation

• go test ./pkg/gui/modes/filtering ./pkg/gui/controllers/...

All tests passed.